I went with the Duraspark system in mine, works great and never had a problem with the install. I think mustangs unlimited makes a wiring harness for the conversion if you don't want to cut your pretty three prong connectors.
If you want to go quick and dirty, get a four-pin HEI module for $20 from the parts store. The Duraspark is bigger and better, though. I think they have an engine cranking input wire that pulls timing until the engine starts, which makes life a little easier on the starter.